This invention relates to mass rate of flow meters of the angular momentum type having a swirl generator for imparting swirl to the measured fluid stream and a torque balance reaction generator for removing the imparted swirl. More particularly, this invention relates to such a meter having an improved readout system for indicating the mass rate of flow.
This invention is particularly adapted for use in a mass rate of flow meter which utilizes a spring-restrained turbine as the torque balance reaction generator. One such mass rate of flow meter is depicted in U.S. Pat. No. 4,056,976 issued Nov. 8, 1977 and titled Mass Rate of Flow Meter, which patent is assigned to the same assignee as the present invention. This meter includes a housing that defines a fluid passage that extends along a longitudinal axis through the housing and that has an input port and an output port located on the axis. A swirl generator is located adjacent the input port to impart a substantially constant angular velocity to an entering fluid stream. As the fluid leaves the swirl generator, it passes through an axially displaced, unrestrained rotor that rotates about the axis. The angular velocity of the rotor accurately represents the angular velocity of the fluid stream as it leaves the rotor and passes through an axially spaced, spring-restrained turbine. The angular momentum of the fluid stream angularly displaces the turbine about the axis and against the bias of its restraining spring. Under steady state conditions, this deflection of the turbine is proportional to the mass rate of flow.
In a spring-restrained flow meter, the rotor carries two circumferentially and longitudinally displaced bar magnets. The first magnet is disposed on the input end of the rotor and is circumferentially poled. A first sensing coil assembly in a transverse plane through the first magnet is radially spaced from the magnet and isolated from the fluid flow. Each time the first magnet passes the first sensing coil, it induces a "start" pulse in the coil that indicates the passage of a predetermined point on the rotor past a predetermined point on the housing.
The second magnet is at the exit end of the rotor and diametrically opposed to the first magnet. An axially disposed, longitudinally extending bar of a highly permeable material, such as soft iron, mounts on the periphery of the turbine. The axial spacing between the rotor and the turbine interposes an axial air gap between the bar and the second magnet when they align. A second sensing coil assembly, that is isolated from the fuel flow, is coaxial with and longitudinally coextensive with the second magnet and the bar. Each time the second magnet passes the bar, the flux that the bar couples to the second sensing coil assembly changes and induces a voltage "stop" pulse in the second sensing coil. As described in the foregoing U.S. Pat. No. 4,056,976, timing circuits convert the start and stop pulses from the first and second sensing coil assemblies into an indication of the mass rate of flow through the meter.
Flowmeters of the type described in U.S. Pat. No. 4,056,976 are normally used in aircraft. In such applications, flowmeters are disposed in an environment that is characterized by high levels of electromagnetic interference. For example, on an aircraft alternators disposed on an engine and a number of conductors around the engine are sources of electromagnetic inerference. It is therefore necessary to construct a flowmeter which is insensitive to this electromagnetic interference. If the flowmeter is susceptible to electromagnetic interference, it can produce a signal even while there is is no flow and no motion of the rotor.
During operation, the rotor undergoes a wide range of rotational velocities. As the voltage induced in the coils is proportional to that velocity, the magnitude of the voltages that are produced in the sensing coils varies. In one particular application, for example, the voltage amplitudes of the pulses produced over a normally encountered range of fuel flows varies from about 30 millivolts to about 500 millivolts. With such variations it is necessary to minimize the signals that any electromagnetic interference might produce to a level below the 30 millivolt level, that is, that minimum voltage that would normally be generated during operation.
The first sensing coil has a longitudinal axis that extends radially with respect to the housing. The coil is a compact coil mounted on the housing of the flowmeter. It is relatively easy to shield from electromagnetic interference, as by the use of a highly permeable, magnetic cover that isolates the coil from the effects of electromagnetic interference. However, if that approach is attempted with the second sensing coil, which is wrapped completely around the housing, it does not produce acceptable results. It does not completely shield the coil from electromagnetic interference. The material that would be needed to provide acceptable results would add weight to the flowmeter which is not desirable in an aircraft application.
Another approach that might be used is that of compensating the electromagnetic interference by adding a coil that would produce a voltage of equal, but offsetting, value to that produced in the sensing coil in response to electromagnetic interference. Such coils are commonly known as a "bucking" coils. The bucking coil might be disposed either in parallel or in series with the sensing coil. However, it has been found that electromagnetic interference apparently induces a current in the housing itself which is composed of an electrically conducting material. This produces a phase shift between the voltages induced in the sensing coil and in the bucking coil. Thus, while the net effect is to reduce the signals that are induced by electromagnetic interference, the minimum levels that are achieved are still greater than acceptable levels.